FEATURE: AI artifacts

[SamSaffron]

This is a significant PR that introduces AI Artifacts functionality to the discourse-ai plugin along with several other improvements. Here are the key changes:

1. AI Artifacts System:

   • Adds a new AiArtifact model and database migration
   • Allows creation of web artifacts with HTML, CSS, and JavaScript content
   • Introduces security settings (strict, lax, disabled) for controlling artifact execution
   • Implements artifact rendering in iframes with sandbox protection
   • New CreateArtifact tool for AI to generate interactive content

2. Tool System Improvements:

   • Adds support for partial tool calls, allowing incremental updates during generation
   • Better handling of tool call states and progress tracking
   • Improved XML tool processing with CDATA support
   • Fixes for tool parameter handling and duplicate invocations

3. LLM Provider Updates:

   • Updates for Anthropic Claude models with correct token limits
   • Adds support for native/XML tool modes in Gemini integration
   • Adds new model configurations including Llama 3.1 models
   • Improvements to streaming response handling

4. UI Enhancements:

   • New artifact viewer component with expand/collapse functionality
   • Security controls for artifact execution (click-to-run in strict mode)
   • Improved dialog and response handling
   • Better error management for tool execution

5. Security Improvements:

   • Sandbox controls for artifact execution
   • Public/private artifact sharing controls
   • Security settings to control artifact behavior
   • CSP and frame-options handling for artifacts

6. Technical Improvements:

   • Better post streaming implementation
   • Improved error handling in completions
   • Better memory management for partial tool calls
   • Enhanced testing coverage

7. Configuration:

   • New site settings for artifact security
   • Extended LLM model configurations
   • Additional tool configuration options

This PR significantly enhances the plugin's capabilities for generating and displaying interactive content while maintaining security and providing flexible configuration options for administrators.